He took the Great Ultimate (Taiji) and the one qi before Heaven to be the root from which the myriad beings stemmed, and which in human beings formed inner nature (xing) and destiny or life (ming). He then advocated the dual cultivation of inner nature and life
This passage provides the most explicit technical definition in the corpus, identifying inner nature (xing) as the human manifestation of the cosmological Great Ultimate, and establishing its inseparability from destiny/life-force (ming) as objects of alchemical cultivation.
, Daoism Handbook, 2000thesis